Xylans are heteropolysaccharides which form the major part of the hemicellulose present in the plant biomass.
The backbone of these polysaccharides is a chain of β-1,4 linked xylopyranosyl residues. Many different side groups could bind to these residues like acetyl, arabinosyl and glucuronosyl residues. Phenolic compounds such as ferulic or hydroxycinnamic acids are also involved through ester binding in the cross linking of the xylan chains or in the linkage between xylan and lignin chains for example.
Endoxylanases hydrolyze specifically the backbone of the hemicellulose. In some cases, the side groups may mask the main chain by steric hindrance. Different xylanase activities already described are characterized by their specificity towards their substrate and the length of the oligomers produced.
These differences between the xylanases concerning their properties seem to be partly related to their respective amino acid sequences. Endoxylanases have been classified into two families (F or 10 and G or 11) according to their sequence similarities (Henrissat & Bairoch 1993) Biochem. J. 293:781). The F family of xylanases are larger, more complex as compared to the G family of xylanases. Moreover the F family xylanases produce small oligosaccharides, while the G family xylanases show a higher affinity for unsubstituted xylan.
Xylanases are used in various industrial areas such as the pulp, paper, feed and bakery industries. Other applications include the juice and beer industries. Xylanases could also be used in the wheat separation process. The observed technological effects are, among others, improved bleachability of the pulp, decreased viscosity of the feed or changes in dough characteristics.
Many different microbial genera have been described to produce one or several xylanases. These microbial genera comprise bacteria as well as eukaryotic organisms like yeast or fungi.